1. Field of the Invention
This invention relates to control devices for movable lenses and, more particularly, to control devices for zoom lenses.
2. Description of the Related Art
Zoom lenses having (1) a focusing function, and (2) a function for compensating for the image shift during zooming, where both functions are performed by one and the same lens group or compensator, have been proposed, for example, in Japanese Patent Publication No. Sho 52-15226 or Japanese Patent Publicatin No. Sho 56-47533. In such a zoom lens, unlike the zoom lens of the type in which the above-described two functions are performed by respective individual lens groups, it becomes possible to minimize the size of the lens system and to shorten the minimum object distance.
In the above-mentioned Japanese Patent Publication No. Sho 52-15226, based on the position information of the variator lens group, a position to which the compensator lens group has to move is determined by the computing treatment.
Meanwhile, in the above-mentioned Japanese Patent Publication No. Sho 56-47533, the defocusing resulting from the change of the image magnification is sensed by the automatic focus detector to effect the adjustment of the compensator lens group.
By the way, the inventions of the above-mentioned publications would have no problem if the aforesaid treatment were performed in real time. But, it takes a good deal of time. Therefore, particularly when zooming, immediate response of the compensator to the actuation of the variator cannot be permitted to occur, thereby making it difficult to sustain the in-focus state.
On this account, Japanese Laid-Open Patent Application No. Sho 62-296110 discloses a technique of compensating for this response delay of the automatic focus detector. According to that document, a plurality of zones defined by the position relationship of the variator lens and the compensator lens are set, and a unit movement amount of the compensator lens for every one of these zones corresponding to a predetermined movement amount of the variator lens is memorized. Then, on the basis of the position information of the variator lens and the compensator lens at the time when a predetermined movement of the variator lens has been detected, a unit movement amount of the compensator lens is obtained according to the zone to be set by this position information. Further, on the basis of the direction of zooming and the information of blur detected by the automatic focus adjusting device, the above-described unit movement amount is corrected. Accoring to this corrected movement amount, the above-described compensator lens is moved. Thus, the compensation is effected.
However, in such a control method as has been described above, even though, when zooming from the telephoto side to the wide-angle side, a relatively good result is obtained, there is a problem that when zooming from the wide-angle side to the telephoto side, the operation is apt to be unstable and the blur cannot sufficiently be corrected. On the other hand, in order not to produce a blur, a speed of zooming has to be considerably slow.
The above-described problem seems to be attributable to the depth of field. In more detail, while the depth of field is deep on the wide-angle side, the depth of field is shallow on the telephoto side. For zooming from the telephoto side to the wide-angle side, if a blur on the telephoto side lies within the acceptable circle of confusion, the deviation from the accurate zoom curve, even if somewhat larger than the tolerance, causes blurring not to appear with, because the depth of field deepens as the focal length shortens. However, for zooming from the wide-angle side to the telephoto side, even if the blur at the start lies within the acceptable circle of confusion due to the deep depth of field, even a slight deviation from the true zoom curve causes a blurring to appear, as the focal length increases.
Here, it may be considered that the response speed of the automatic focus detection is increased, or the above-described number of divided zones is sufficiently large to correct the blur with a higher accuracy. Yet, there is a limitation. Another problem of cost will arise.